It is known to periodically sample the alternating voltage and the alternating current delivered to a load to digitally measure the sampled points, to multiply the sampled points corresponding to the voltage and current, respectively, and to integrate the multiplied values so as to determine the power consumed by the load. Although the average value of an alternating waveform, be it a current or voltage waveform, is zero, the sum of periodically sampled values of an alternating current or voltage waveform is not necessarily zero, but only approaches zero; the deviation from the zero value of the sum of the samples of an alternating current or voltage waveform causes finite errors in the power measurement, when the aforesaid digital approach is used.
The deviation from zero is caused by such factors as asymmetry of the sampled points of the positive and negative portion of the waveform, respectively, current or voltage spikes, superimposed direct currents or voltages, and other causes.
An at least partial solution to the problem has been indicated in Swiss Pat. No. CH-PS 538,122 corresponding to British Pat. No. GB-PS 1,361,140, in which a product between current and voltage is formed according to a statistical coincidence method. In the Journal "Alta Frequenza" Vol. 35, Number 11, pages 889 to 898, Sacerdoti et al. disclose a digital energy meter, based on the sum of the partial product of instantaneous voltage and current values.
From German Pat. No. DE-27 14 632, there has become known a method for determining electrical power consumed by a load, wherein a stochastic coder employs two null potential scanning phases within an interrogation cycle, so as to form the product of null potentials, an impulse sequence proportional to this product being counted by forward/backward counter, when counting in the backward mode.
Richman, in U.S. Pat. No. 3,778,608 discloses electrical measuring systems using a multiplier operating according to the formula (a+b).sup.2 -(a-b).sup.2 +4ab.
Hunter, in British Pat. No. 1,373,581 discloses a wattmeter using an analog to digital converter, and a digital multiplier.
None of the aforesaid references accomplish the task of compensating for any deviation of the average from zero of digitized values of a voltage or current waveform in a simple manner.